The present invention relates to a noise attenuation panel comprising a first wall, a second wall and partition walls connected to the first and second walls and defining cells between the first and second walls, wherein the first wall is provided with a plurality of through holes. The invention is further directed to a gas turbine component comprising a noise attenuation panel.
The gas turbine component is configured for application in a gas turbine engine, and especially in an aircraft engine. Thus, the invention is especially directed to a jet engine. Jet engine is meant to include various types of engines, which admit air at relatively low velocity, heat it by combustion and shoot it out at a much higher velocity. Accommodated within the term jet engine are, for example, turbojet engines and turbofan engines. The invention will below be described for a turbofan-engine, but may of course also be used for other engine types.
Said noise attenuation panel is used to reduce the noise of the engine and the fan in operation. Further, the partition walls are often designed in a honeycomb pattern. An advantage with a honeycomb cell type structure is that it is structurally stable. In certain cases, the cells of the honeycomb material act as a Helmholtz resonator when they communicate with the outside. This provides an-acoustic panel which absorbs the noise over a certain range of frequencies.
The Helmholtz type of liner (panel) is well known for its good sound suppressing qualities. Its impedance may be regarded as a mass-spring-damper system where the air in the neck serves as the mass, the volume of air in the body as a spring and viscous forces in the neck (through hole) together with nonlinear effects serves as the damping. The drawback of this type of liner is that the resonance frequency is determined, by the ratio between the cross-area/depth ratio of the neck and the volume of the body. This means that in order to make a low frequency liner the neck must have a small diameter and a great depth in combination with a large body volume.
The noise is primarily generated between the fan and an arrangement of outlet guide vanes positioned downstream of the fan. It is for example known to use said noise attenuation panel in a circumferential inner wall defining the air inlet upstream of the fan.
The noise attenuation panel is designed depending on the aircraft engine type and design and a desire to attenuate noise in certain operations. For example, there is often a desire to attenuate noise during takeoff. Further, one effect of some modern aircraft engines is that noise of lower frequencies than before is generated during said defined operation. Further, there is a desire to use sound attenuation panels in more positions in the aircraft engine.
It is desirable to achieve a noise attenuation panel, which is configured to attenuate low frequency tonal noise with a minimal depth requirement.
According to an aspect of the present invention, in a noise attenuation panel at least two of said cells are interconnected via a communication hole, that one of said through holes leads to a first of said at least two interconnected cells and that a second of said interconnected cells is configured to prevent any gas flow through the, second cell. The communication hole is preferably provided in the partition wall between the cells
Thus, the panel is configured to allow a gas flow through the first cell via said through hole and said communication hole. Further, said second cell is closed and is only communicating with the surroundings via said communication hole. In other words, the part of the first wall defining the second cell is devoid of through holes.
More specifically, both a part of the first wall defining the second cell and a part of the second wall defining the second cell are continuous thereby forming closed opposite sides of the second cell. Further, the second cell is only in communication with an external environment via a single adjacent cell of said interconnected cells via the communication hole. The communication hole is preferably arranged in a partition wall in common with said adjacent cell.
In this way, the panel acts as a series of helmholtz resonators with a theoretical resonance frequency of a quarter wavelength. This is not the full physicality due to a small modification of the resonant frequency due to the spring-mass-damper system of the cells. This slows down the reaction of the system, additionally lowering the resonance frequency.
The resonance frequency of the panel will be lower for a cell structure with a specific depth. More specifically, the resonance frequency of the panel depends on an efficient distance from die through hole (inlet hole) to an fend of the interconnected cells. By interconnecting a plurality of cells in series and designing a last cell (said second cell) to prevent any gas flow therethrough, the resonance frequency is lowered for a cell structure with a specific-depth. This design creates conditions for designing a relatively thin panel and thereby using the panel in positions with space constraints, such as in a guide vane.
In other words, the noise attenuation panel is generally composed of two metal or composite walls or skins and a central section made from metal or composite cellular material.
According to a preferred embodiment, said-partition walls-form a honeycomb structure. In this way, a structurally stable design is achieved, which creates conditions for, withstanding the pressure conditions at its defined position in the gas turbine engine.
According to a further preferred embodiment; at least three cells are interconnected via communication holes.
This creates conditions for achieving a longer distance between the inlet hole (the through hole in the first wall) to the first cell and an end wall in the last cell (the second cell) in the series of interconnected cells.
According to a development of the last-mentioned embodiment, the first cell forms an end cell in said at least three interconnected cells. In this way, a longer distance is achieved between the inlet hole (through hole in the first wall) to the first cell and an end wall in the last cell (the second cell) in the series of interconnected cells.
According to a further development of the last-mentioned embodiment, at least three cells of said at least three interconnected cells are arranged along a straight line. This design further creates conditions for achieving a minimized resonance frequency.
According to a further development of the last-mentioned embodiment, only the first cell in said at least three interconnected cells is provided with a through hole in its associated part of the first wall. This design further creates conditions for achieving a longer distance between the inlet hole (through hole in the first wall) to the first cell and an end wall in the last cell (the second cell) in the series of interconnected cells.
According to a further preferred embodiment, one of said communication holes to the first cell is provided at the opposite end of the partition wall in relation to the first wall. This design further creates conditions for achieving a longer effective distance between the inlet hole (through hole in the first wall) to the first cell and an end wall in the last cell (the second cell) in the series of interconnected cells.
According to a further development of the last-mentioned embodiment, one of said communication holes is provided between the second cell and an intermediate cell between the first and second cells in the interconnected cells in the vicinity of the first wall. This design further creates conditions for achieving a longer distance between the inlet hole (through hole in the first wall) to the first cell and an end wall in the last cell in the series of interconnected cells. More specifically, a second communication hole in the cell series is arranged diagonally opposite the first communication hole in the series. By positioning the communication holes as far apart as possible the time for pressure information to travel is maximized, hence the resonance frequency is minimized. Further, since each passage increases the losses, the damping is increased. Thus, the noise will be more damped with a higher number of communication holes. Thus, the fact that there are communication holes at at least two adjacent partition walls implies that the damping is stronger.
It is also desirable to achieve a gas turbine engine component, which is configured to attenuate low frequency noise.
According to another aspect of the present invention, a gas turbine component comprises at least one noise attenuation panel, wherein the panel comprises a first wall and partition walls connected to the first wall and defining cells, wherein the first wall is provided with a plurality of through holes, characterized in that at least two of said cells are interconnected via a communication hole, that one of said through holes leads to a first of said at least two interconnected cells, and that a second of said interconnected cells is configured to prevent any gas flow through the second cell.
Other advantageous features and functions of various embodiments of the invention are set forth in the following description and in the dependent claims.